Analysis, design and implementation of high performance control schemes in renewable energy source based DC AC inverter for micro grid application

ANALYSIS, DESIGN AND IMPLEMENTATION OF
HIGH PERFORMANCE CONTROL SCHEMES IN
RENEWABLE ENERGY SOURCE BASED DC/AC
INVERTER FOR MICRO-GRID APPLICATION
SOUVIK DASGUPTA
NATIONAL UNIVERSITY OF SINGAPORE
2011
ANALYSIS, DESIGN AND IMPLEMENTATION OF
HIGH PERFORMANCE CONTROL SCHEMES IN
RENEWABLE ENERGY SOURCE BASED DC/AC
INVERTER FOR MICRO-GRID APPLICATION
SOUVIK DASGUPTA
(M.Engg., Bengal Engineering and Science University, India)
(B.Engg.(Hons.), Jadavpur University, India)
A THESIS SUBMITTED
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
DEPARTMENT OF ELECTRICAL AND COMPUTER
ENGINEERING
NATIONAL UNIVERSITY OF SINGAPORE
2011
Acknowledgments
The author wishes to record his deep sense of gratitude to his supervisor, Assoc.
Prof. Sanjib Kumar Panda, who has introduced the present area of work and guided
in this work. The author’s thesis supervisor, Assoc. Prof. Sanjib Kumar Panda
has been a source of incessant encouragement and patient guidance throughout the
thesis work. The author is extremely grateful and obliged to Dr. Sanjib Kumar
Sahoo for his intellectual innovative and highly investigative guidance in the thesis
work. The author likes to express special thanks to Prof. Jian-Xin Xu for his
valuable help in control theory and application. The author is also indebted to
Assoc. Prof. Ramesh Oruganti and Assoc. Prof. Ashwin M Khambadkone for their
incredible teachings in the design aspects of power electronics and drive systems.

The author would also like to thank Assoc. Prof. Thong T. L. John, Assoc. Prof.
Y. C. Liang and Assist. Prof. Akshay K. Rathore for their guidance as PhD
Thesis Committee Members. The author wishes to express his thanks to Mr. Y.
C. Woo, and Mr. M. Chandra of Electrical machines and Drives lab, NUS, for their
readiness to help on any matter. The author is also grateful to his fellow research
scholars, specially Mr. Parikshit Yadav, Mr. Sangit Sasidhar and Mr. Hoang
Duc Chinh, for their constructive criticism in different aspects of this thesis. The
author wishes to convey special thanks to Dr. Xinhui Wu, Dr. Haihua Zhou, Dr.
Yenkheng Tan and Dr. Prasanna U. R. for their inspiring comments whenever the
i
ii
author approached to them.
Last but not the least, the author is strongly indebted to the Almighty for
presenting him the best parents of the whole of Universe. The author’s father
Mr. Sankar Dasgupta and the author’s mother Mrs. Mamata Dasgupta have been
bearing with him in different aspects of life for long time. The author wishes to
dedicate this thesis to their love and support.
Contents
Summary xix
List of Tables xxi
List of Figures xxii
Acronyms xlii
Symbols xliv
1 Introduction 1
1.1 General Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Configuration of a typical multi-bus micro-grid system . . . . . . . 4
iii
Contents iv
1.3 Different topologies of DC/AC inverters and controls to interface
renewable energy sources to the micro-grid . . . . . . . . . . . . . . 7

1.4 Problem Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
1.4.1 Inverters for single-phase residential micro-grid . . . . . . . . 21
1.4.2 Inverters for three-phase industrial micro-grid . . . . . . . . 23
1.5 Literature Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
1.6 Contribution of this thesis . . . . . . . . . . . . . . . . . . . . . . . 34
1.7 Organization of this Thesis . . . . . . . . . . . . . . . . . . . . . . . 36
1.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2 Mathematical model, active and reactive power flow control of
single-phase parallel connected renewable energy source based in-
verter 41
2.1 Description of the inverter configuration and its control . . . . . . . 42
2.1.1 Description of the inverter assembly . . . . . . . . . . . . . . 42
2.1.2 Control strategy of the inverter . . . . . . . . . . . . . . . . 43
Contents v
2.2 Modeling of the CCVSI system . . . . . . . . . . . . . . . . . . . . 44
2.3 Deriving the current reference of the inverter . . . . . . . . . . . . . 46
2.3.1 Using conventional single-phase p-q theory . . . . . . . . . . 46
2.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3 Implementation of control strategy for the single-phase parallel
connected renewable energy source based inverter 51
3.1 Design of Non-Linear Control Law based on Lyapunov function . . 52
3.1.1 Determining the Lyapunov function based control law to en-
sure current control . . . . . . . . . . . . . . . . . . . . . . . 52
3.1.2 Estimation of the disturbance term ‘d’ to facilitate the con-
trol action . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.1.3 Ensuring the stability of the plugged-in Spatial Repetitive
Controller in parallel with the Lyapunov Function based con-
troller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.1.4 Effect of parameter uncertainty on the convergence . . . . . 56
3.1.5 Design of the Lyapunov Function based control law, u

lf
(t) . 57
Contents vi
3.1.6 Design of the Spatial Repetitive Controller based disturbance
estimation control law, u
src
(t) . . . . . . . . . . . . . . . . . 58
3.1.7 Implementation of the proposed control system . . . . . . . 62
3.2 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . 64
3.2.1 Steady-state experimental waveforms . . . . . . . . . . . . . 64
3.2.2 Experimental waveforms to show the transients associated
with Lyapunov Function based controller . . . . . . . . . . . 68
3.2.3 Experimental waveforms to show the transients associated
with plugged-in Spatial Repetitive controller . . . . . . . . . 70
3.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
4 Voltage regulation and active power flow control of single-phase
series connected renewable energy source based inverter 73
4.1 Description of the inverter configuration and its control strategy . . 74
4.1.1 Description of the power circuit of the series inverter . . . . 74
4.1.2 Control strategy of the series inverter under common oper-
ating conditions . . . . . . . . . . . . . . . . . . . . . . . . . 75
Contents vii
4.1.3 Constraint on the series inverter system under common op-
erating conditions . . . . . . . . . . . . . . . . . . . . . . . . 78
4.2 Design of a typical prototype system . . . . . . . . . . . . . . . . . 81
4.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
5 Implementation of control strategy for the single-phase series con-
nected renewable energy source based inverter 84
5.1 Design of Spatial Repetitive Controller . . . . . . . . . . . . . . . . 85
5.1.1 General discussion on Spatial Repetitive Controller . . . . . 85

5.1.2 Position domain modeling of the inverter L-C filter assembly
with load and micro-grid interconnection . . . . . . . . . . . 93
5.1.3 Position domain modeling of the anti-alias filter . . . . . . . 95
5.1.4 Design of the Spatial Repetitive Controller for the series in-
verter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
5.2 Experimental Results of the proposed series inverter system with
Spatial Repetitive Controller operation . . . . . . . . . . . . . . . . 100
5.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
Contents viii
6 Analysis and control of a three-phase renewable energy source
based inverter connected to a generalized micro-grid system 110
6.1 General description of the renewable energy source based inverter:
interface between micro-grid and utility grid . . . . . . . . . . . . . 111
6.1.1 Description of the inverter interaction with the micro-grid . 111
6.1.2 Control methodology of the inverter current . . . . . . . . . 112
6.2 State-space modeling of the three-phase unbalanced grid connected
inverter in the a-b-c frame . . . . . . . . . . . . . . . . . . . . . . . 113
6.3 Design of Non-Linear Control Law based on Lyapunov Function . . 116
6.3.1 Determining the Lyapunov function based control law to en-
sure current control . . . . . . . . . . . . . . . . . . . . . . . 116
6.3.2 Estimation of the disturbance terms d
1
and d
2
to facilitate
successful current tracking . . . . . . . . . . . . . . . . . . . 117
6.3.3 Ensuring the stability of the plugged-in spatial repetitive con-
troller in parallel with the Lyapunov function based controller 118
6.3.4 Effect of parameter uncertainty on the error convergence . . 119
6.4 Implementation of the Lyapunov function based controller . . . . . 121

Contents ix
6.4.1 Implementation of the proposed control system using the
four-switch (b-4) inverter power circuit . . . . . . . . . . . . 121
6.4.2 Implementation of the proposed control system using the six-
switch (b-6) inverter power circuit . . . . . . . . . . . . . . . 122
6.4.3 Implementation of the proposed controller in digital system . 124
6.5 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . 127
6.5.1 Hardware details of the experimental power circuit . . . . . 127
6.5.2 Steady state results for the b-6 topology of there-phase inverter129
6.5.3 Transient results for the proposed control system . . . . . . 133
6.5.4 THD reduction capability of the proposed control system . . 135
6.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
7 Derivation of instantaneous current references for multi-phase PWM
inverter to control active and reactive power flow from a renewable
energy source to a generalized multi-phase micro-grid system 140
7.1 General description of the load and inverter interface with the grid . 141
7.2 p-q theory based CCVSI current reference derivation . . . . . . . . 143
Contents x
7.2.1 p-q theory based current references generation scheme . . . . 143
7.2.2 Implementation of the p-q theory based CCVSI current ref-
erence calculation method . . . . . . . . . . . . . . . . . . . 146
7.2.2.1 BLOCK-A . . . . . . . . . . . . . . . . . . . . . . 148
7.2.2.2 BLOCK-B . . . . . . . . . . . . . . . . . . . . . . . 148
7.2.2.3 BLOCK-C . . . . . . . . . . . . . . . . . . . . . . . 150
7.2.2.4 BLOCK-D . . . . . . . . . . . . . . . . . . . . . . 150
7.2.2.5 BLOCK-E . . . . . . . . . . . . . . . . . . . . . . . 152
7.2.2.6 BLOCK-F . . . . . . . . . . . . . . . . . . . . . . . 152
7.3 FBD theory based CCVSI current reference derivation . . . . . . . 153
7.3.1 FBD theory based current reference generation scheme . . . 153
7.3.2 Implementation of the FBD theory based CCVSI current

reference calculation method . . . . . . . . . . . . . . . . . . 155
7.3.2.1 BLOCK-A to BLOCK-D . . . . . . . . . . . . . . . 155
7.3.2.2 BLOCK-E . . . . . . . . . . . . . . . . . . . . . . . 157
Contents xi
7.3.2.3 BLOCK-F . . . . . . . . . . . . . . . . . . . . . . . 157
7.3.2.4 BLOCK-G . . . . . . . . . . . . . . . . . . . . . . 157
7.4 Instantaneous power theory based CCVSI current reference derivation158
7.4.1 Calculation of instantaneous power for an ‘n’-phase grid con-
nected system . . . . . . . . . . . . . . . . . . . . . . . . . . 158
7.4.2 Extracting the solution provided by p-q theory . . . . . . . . 161
7.4.3 Analysis of the solution provided by FBD theory . . . . . . 162
7.4.4 Derivation of the grid current reference for a typical three
phase unbalance system (n = 3) . . . . . . . . . . . . . . . 164
7.5 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . 165
7.5.1 Experimental results to show the performance of the complex
notch filter . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
7.5.2 Experimental results to show the reference current generation
using p-q and FBD theory . . . . . . . . . . . . . . . . . . . 168
7.5.3 Experimental results to show the grid current tracking for
the CCVSI . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
Contents xii
7.5.4 Experimental results to show the DC link ripple comparison
in p-q and FBD theory based grid current estimation . . . . 172
7.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
8 Application of four-switch based three-phase grid connected in-
verter to connect renewable energy source to a generalized micro-
grid system 178
8.1 Four switch three-phase VSI (b-4 topology) based grid connected
inverter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
8.2 Mathematical modeling of the b-4 topology based grid connected

inverter and description of the control structure . . . . . . . . . . . 180
8.3 Implementation of the control system for the b-4 topology based
grid connected inverter . . . . . . . . . . . . . . . . . . . . . . . . . 183
8.4 Effect of DC link split capacitor unbalance on the operation of the
b-4 topology three-phase inverter for grid connected application . . 188
8.5 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . 190
8.5.1 Experimental results to show the operation of the b-4 topol-
ogy based three-phase inverter in the presence of non-linear
load at the grid terminals . . . . . . . . . . . . . . . . . . . 191
Contents xiii
8.5.2 Experimental results to show the operation of the b-4 topol-
ogy based three-phase inverter sinking power to grid . . . . 197
8.5.3 Experimental results to show the effect of the control system
on the DC link split capacitor unbalance for the b-4 topology
based three-phase inverter . . . . . . . . . . . . . . . . . . . 200
8.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203
9 Conclusions and Future Work 204
9.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
9.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210
Bibliography 212
Papers and patents 230
A 237
1.1 Photo of the implemented single-phase and three-phase in-
verter systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237
B 241
Contents xiv
2.1 Maximum Power Point operation at the presence of bat-
tery in the inverter DC link . . . . . . . . . . . . . . . . . . . 241
C 245
3.1 DC link voltage control of the PV inverter at the absence

of the storage element in the DC link . . . . . . . . . . . . . 245
D 248
4.1 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . 248
4.1.1 Three phase programmable AC power supply . . . . . . . . 250
4.1.2 Digital controller for implementation the control system . . . 251
4.1.2.1 Hardware Features . . . . . . . . . . . . . . . . . . 251
4.1.2.2 Software Features . . . . . . . . . . . . . . . . . . . 253
4.1.3 Power Converter and Driver . . . . . . . . . . . . . . . . . . 254
4.1.4 Voltage and current sensors . . . . . . . . . . . . . . . . . . 255
4.1.5 Signal interface board . . . . . . . . . . . . . . . . . . . . . 256
4.1.6 Programmable DC power supply . . . . . . . . . . . . . . . 257
Contents xv
4.1.7 Load arrangements . . . . . . . . . . . . . . . . . . . . . . . 257
E 259
5.1 Using modified single-phase p-q theory . . . . . . . . . . . . 259
F 263
6.1 Control strategies for series inverter to pump active power
to grid and charging DC link battery . . . . . . . . . . . . . . 263
6.1.1 Control strategy of the inverter to feed power to the grid . . 263
6.1.2 Control strategy of the inverter to store the power from the
grid in the DC link battery . . . . . . . . . . . . . . . . . . 264
6.1.2.1 Charging battery when there is voltage sag in grid 266
6.1.2.2 Charging the battery when there is voltage swell as
well as normal condition of grid . . . . . . . . . . . 267
G 268
7.1 Designing Lyapunov function based controller for series
inverter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268
Contents xvi
7.1.1 Deriving the state-space representation of the series inverter 268
7.1.2 Designing the Lyapunov function based controller . . . . . . 269

7.1.2.1 Considering the case of d = 0 and the values of the
parameters of the system are known perfectly . . . 270
7.1.2.2 Considering the presence of disturbance d = 0 with
parameter uncertainty of the system . . . . . . . . 271
7.1.2.3 Finite time reaching property of the Lyapunov func-
tion based sliding mode control action . . . . . . . 272
7.1.2.4 Steady state equation of the states of the system . 273
7.2 Experimental results of the proposed series inverter system with Lya-
punov function based controller operation . . . . . . . . . . . . . . 274
7.2.1 Testing of the tracking capability of the Lyapunov function
based controller in the basic power circuit without disturbance275
7.2.2 Testing of the tracking capability of the Lyapunov function
based controller in the series inverter with grid . . . . . . . . 278
H 285
Contents xvii
8.1 Comparison of the performance of the proposed Lyapunov
function based controller and the traditional PI+fundamental
frame multiple PR controller for three-phase generalized
grid connected CCVSI . . . . . . . . . . . . . . . . . . . . . . . 285
8.2 Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 285
I 288
9.1 Brief description of main contributions of this thesis . . . 288
9.1.1 Control methodology of single-phase parallel connected re-
newable energy source based inverter connecting to micro-
grid to control active and reactive power flow with grid cur-
rent shaping . . . . . . . . . . . . . . . . . . . . . . . . . . . 288
9.1.2 Control methodology of single-phase series connected renew-
able energy source based inverter connecting to micro-grid
to mitigate voltage related problems along with active power
flow control . . . . . . . . . . . . . . . . . . . . . . . . . . . 289

9.1.3 A Lyapunov function based current controller to control ac-
tive and reactive power flow from a renewable energy source
to a generalized three-phase micro-grid system . . . . . . . . 291
Contents xviii
9.1.4 Derivation of instantaneous current references for multi-phase
PWM inverter to control active and reactive power flow from
a renewable energy source to a generalized multi-phase micro-
grid system: the p-q theory based approach . . . . . . . . . 292
9.1.5 Derivation of instantaneous current references for multi-phase
PWM inverter to control active and reactive power flow from
a renewable energy source to a generalized multi-phase micro-
grid system: the FBD theory based approach . . . . . . . . 293
9.1.6 Application of four-switch based three-phase grid connected
inverter to connect renewable energy source to a generalized
unbalanced micro-grid system . . . . . . . . . . . . . . . . . 294
Appendices 237
Summary
In traditional micro-grid application, harvested renewable energy is interfaced with
the single/three-phase micro-grid using single(typical residential application)/three(typical
industrial application)-phase power electronic converters/inverters (Distributed gen-
erators or DGs). Power flow control as well as shaping of the current drawn from
the common AC bus (grid) of the micro-grid is primarily done by controlling the
inverter currents using suitable current references, which in turn necessitates digi-
tally implemented high-performance controllers for these applications. This thesis
investigates different high-performance control schemes to control power flow as
well as shaping voltages/currents under different adverse operating conditions in
the micro-grid.
In the first part of the thesis, a Lyapunov function based current controller
is proposed for a single-phase parallel connected inverter along with a local load
connection. The proposed control system ensures high-performance tracking of the

inverter current derived by single phase p-q theory to ensure a specific amount of
active and reactive grid power consumption by the load along with maintaining
grid current to be sinusoid. The proposed controller gives superior performance
over conventional PI + resonant controller. In the second part of the thesis, a
single-phase series connection of the DG inverter along with micro-grid and load
xix
Summary xx
is proposed. The proposed method ensures rated high quality of the load voltage
even in the presence of sag, swell or harmonic distortions in the micro-grid volt-
age, using a Spatial Repetitive Controller(SRC), facilitating micro-grid fundamen-
tal frequency independent performance. The total load active power is controllably
shared between inverter and micro-grid with the assurance of leading micro-grid
power-factor even if the load power-factor is lagging. In the last part of the thesis,
DG inverter connection is considered in parallel to a generalized three-phase micro-
grid along with local load. Controllable load power sharing with the control on the
grid current THD is also ensured with a proposed Lyapunov function based current
controller. The proposed method considers unbalance not only in the grid voltages
but also in the line side inductances while the controller is implemented in a-b-c
frame. The three-phase p-q theory and FBD theory based approaches are used to
calculate the inverter current reference and the corresponding effects on DC link
side ripples are also investigated. A Complex Notch Filter (CNF) based approach is
proposed to extract fundamental positive as well as negative sequence components
from the generalized grid voltages for the a-b-c frame implementation of the p-q
theory based approach. The proposed FBD based approach is implemented on the
grid fundamental phase domain ensuing high-performance operation even under
fractional change in grid frequency. Both b-6 (six-switch) and b-4 (four-switch)
three-phase inverter topologies are tested for such DG interconnection. The pro-
posed control technique ensures simple Sine PWM based control of b-4 inverter
unlike the conventional adaptive SVPWM method. Detailed experimental results
are provided to show the efficacy of each of the methodologies.

List of Tables
3.1 Parameters of the experimental power circuit . . . . . . . . . . . . 65
4.1 Different values of Power Angle, γ . . . . . . . . . . . . . . . . . . 82
6.1 Parameters of the experimental power circuit . . . . . . . . . . . . 128
8.1 Parameters of the experimental power circuit . . . . . . . . . . . . 194
xxi
List of Figures
1.1 Typical configuration of inverter-based micro-grid. . . . . . . . . . . 5
1.2 Hardware structure of three phase grid connected PV system [34]. . 9
1.3 General structure for synchronous rotating frame control structure
[34]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.4 General structure for stationary frame control structure [34]. . . . . 10
1.5 (a) One-phase each multi-string converter. (b) Three-phase com-
bined multi-string converter [33]. . . . . . . . . . . . . . . . . . . . 11
1.6 (a) Single-stage inverter (b) Dual power processing inverter, dual-
stage inverter (c) Multi-string inverter [35]. . . . . . . . . . . . . . . 12
1.7 Electrical characteristics of the PV panel and the double harmonic
power oscillation at the panel terminal [35]. . . . . . . . . . . . . . 13
xxii
List of Figures xxiii
1.8 Different location of decoupling capacitor; (a) Single-stage inverter:
capacitor only placed in parallel with the PV panel . (b) Multi-stage
inverter: Capacitor placed in parallel to the PV panel as well as in
the dc -link [35]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.9 Unfolding inverter based single phase PV module inverter system [40]. 15
1.10 Stand alone PV inverter systems. (a) all the loads are AC loads. (b)
there are both AC as well as DC loads [41]. . . . . . . . . . . . . . . 16
1.11 Partial Power Electronic Converter and Wound Rotor Induction
Generator (WRIG) with gear train based wind energy harvester [34]. 17
1.12 Full power Electronic converter and Slip Ring Induction Generator

(SRIG) or Synchronous Generator (SG) with gear train based wind
energy harvester [34]. . . . . . . . . . . . . . . . . . . . . . . . . . . 18
1.13 Full power electronic converter and Permanent Magnet Synchronous
Generator (PMSG) without gear train based wind energy harvester
[34]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
1.14 Configuration of flexible micro-grid with the interaction of different
renewable energy sources [11]. . . . . . . . . . . . . . . . . . . . . . 20
1.15 Illustration of typical PFC circuits. . . . . . . . . . . . . . . . . . . 22
2.1 Power circuit of the single-phase micro-grid connected inverter. . . . 43